In television systems operating according to IP (Internet Protocol) protocol, the video streams are generally transmitted to subscribers via a broadband connection. More specifically, a transmission signal is transmitted to digital decoders (Set Top Box) of subscribers via an access network and DSL (Digital Subscriber Line) modems. This transmission signal is decoded then converted into standard television signals by the digital decoder of the subscriber.
To change channel, the subscriber presses the programme keys P+ or P− of the digital decoder remote control. The P+ key enables incrementing the channel number currently being viewed and the P− key enables decrementing it. In response to a channel change request, the digital decoder transmits to the access network a physical channel change request via its DSL modem. Typically this request is carried out using the IGMP (Internet Group Management Protocol) protocol. In this case, the physical channel change request is a subscription request. The digital decoder then sends as a subscription parameter the IP address of the channel that it wants to receive.
FIG. 1 shows a timing diagram showing the transmission of physical channel change requests by the decoder following a plurality of channel change requests from the subscriber via his remote control according to a standard process of the prior art. When the subscriber presses the P+ or P− key of his remote control, the decoder immediately transmits a physical channel change request to the access network. If the subscriber presses n times successively on the keys P+ or P− of his remote control in a very short time interval to rapidly attain a given channel, n physical channel change requests are transmitted to the access network although only the last of them would suffice as it contains the IP address of the last requested channel. This massive transmission of physical channel change requests creates a useless overloading, even saturation, of the access network.
To avoid this overloading of the access network, it is known in the art to not transmit all the physical channel change requests. This operating mode is shown in FIGS. 2 and 3 and is classically known as virtual zapping. When the digital decoder receives a channel change request (pressure on the P+ or P− key of the remote control), it triggers a timeout for a duration Ttempo. If no new channel change request is received during this timeout, a physical channel change request corresponding to this channel change request is transmitted at the end of this timeout. Inversely, if a new channel change request is received during this timeout, the timeout is reset and the process previously described is recommenced.
FIG. 2 shows a case where several channel change requests are received by the digital decoder during a reduced time period, the time interval between each channel change request being less than the period Ttempo of the timeout. This is then referred to as burst zapping or as a channel change request burst. In the example shown in this figure, the digital decoder receives 4 channel change requests corresponding to four successive and grouped presses on the P+ or P− key of the remote control. In this example, a channel change request is received during the timeout of the first three channel change requests. No physical channel change request is therefore transmitted for these first three channel change requests. Information relating to these channel change requests is however displayed upon their reception on the television screen connected to the digital decoder to inform the subscriber that his requests were indeed received by the decoder. Finally, concerning the fourth channel change request, no other channel change request being received during its timeout period, a physical channel change request corresponding to this channel change request is therefore transmitted to the access network at the end of the timeout period.
FIG. 3 shows a case where several channel change requests are received by the digital decoder over a longer time period, the time interval between each channel change request being greater than Ttempo. This case corresponds to a slow zapping situation in which the subscriber carries out successively several simple zappings. In this situation, the subscriber does not want to go directly to a given channel but wants to view each of the channels corresponding to a press of the P+/P− key. In the case shown in FIG. 3, the digital decoder receives three channel change requests separated from each other by a time interval greater than Ttempo. No channel change request being received during the timeout period of the 3 channel change requests, a physical channel change request corresponding to each of these channel change requests is transmitted at the end of the timeout period that follows the reception of the channel change request.
The operating mode described with respect to FIGS. 2 and 3 enables the transmission of a large number of physical channel change requests to the access network to be avoided and thus avoids an overloading or saturation of this latter. However, the physical channel change requests are transmitted to the access network with a delay equal to the duration of the timeout (Ttempo). The result is that, in the case of a simple zapping, the system loses in reactivity as the subscriber must wait, in addition to the processing time of a request by the network, the time Ttempo before obtaining the channel change on his television. Moreover, this runs counter to the improvement in the service quality required by the operators.